Squib pulsing high current switch



April 12, 1966 s. H. KAUFFMAN 3,246,109

SQUIB PULSING HIGH CURRENT SWITCH Filed Feb. 28, 1964 2 Sheets-Sheet 1FIG. 1

Samuel H. Kauffman INVENTOR.

ATTORNEY AGENT April 12, 1966 s. H. KAUFFMAN SQUIB PULSING HIGH CURRENTSWITCH 2 Sheets-Sheet 2 FIG. 2

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AGENT.

United States Patent 3,246,109 SQUIB PULSING HIGH CURRENT SWITCH SamuelH. Kautfman, Silver Spring, Md., assignor to the United States ofAmerica as represented by the Secretary of the Navy Filed Feb. 28, 1964,Ser. No. 348,910 1 Claim. (Cl. 200166) The invention described hereinmay be manufactured and used by or for the Government of the UnitedStates of America for governmental purposes without the payment of anyroyalities thereon or therefor.

This invention relates to the art of electric switches. Moreparticularly, the invention concerns a switch intended to be used todischarge a capacitor in an explosive squib firing circuit in which arelatively high current for a short time is required. The invention isan improvement over the time delay switch described in U.S. Patent2,998,944 to William Rimmer, issued September 5, 1961.

The present switch is intended for use as a rocket motor igniter switchalthough it is to be understood that it may be used in any high currentfiring circuit. Because of this important military application, theswitch must meet rigorous test requirements. The prescribed performancecharacteristics are set out in Ordnance Standard 9816, paragraph 3.4.8.2as follows:

' Discharge. across one (1) ohm resistance-The measured voltage tracefrom an oscilloscope shall be not less than 90 percent of the voltagefor a perfect switch for the initial two time constants after theinitial rise of the trace except during the first 150 microseconds, whenthe measured voltage shall be not less than 67 percent of the perfectswitch voltage. After the first 150 microseconds, the trace shall besmooth and continuous. The equation for the curve for a perfect switchsignature is given as:

10% VDC-60e ""m (Note that near i=0, the current in the 1 ohm testcircuit is about 60 amps.)

The switch described in the aforementioned Rimmer patent, which hasconventional plane and spherical contact surfaces, was found to beunable to meet this standard. Another contact scheme was tried, using ashorting bar and a pair of spring fingers, to provide a switch having awiping action, but this too failed to meet the requirements of 05-9816.In both cases, the failure was apparently due to arcing at the contacts.

Accordingly, it is the object of this invention to provide a switchwhich exhibits contact signature performance above the minimum standardsset out above.

It is known in the switch art that arcing and high contact resistancesometimes results from the fact that surface roughness makes it possiblefor first contact to be established by a whisker or protuberance fromthe contact surface. If the whisker or protuberance cannot carry thecurrent in the circuit, it vaporizes and provides an ionized path for anarc to be established.

This arcing takes place even where, as in the present case, the voltagesare not high enough to cause spontaneous air arcing. The vaporizationphenomenon is discussed, for example, by Ragner Holm in his bookentitled Electric Contacts at page 280. It is suggested there that sucharcing could be overcome by providing a high polish on the contactsurfaces, but the assertion is also made that it is not possible toachieve satisfactory results in practice. Applicant has found, however,that by making the contacts cylindrical (i.e. circularly cylin drical asdistinguished from generally cylindrical) in shape, it becomes possibleto polish the surfaces easily to a high degree. Then, by arranging thetwo cylinders so that the planes containing their axes are at asubstantial angle to each other, a substantially point contactarrangement is provided in which arcing does not occur.

The cross relationship of the contacts makes known the exact point atwhich contact will take place. Special attention can then be given tothe inspection and polishing of the contacts at this point. While theprecise reason for the good performance of the switch is not known, itis thought that it results from the combination of the features of highpolish, the cylindrical shape of the contacts, and the crossrelationship between them.

A further advantage of the present invention resides in the discoverythat contact can be made repeatedly and at full load without burning thecontacts. Thus the switch can be tested several times before use andreliability of performance in actual use is increased.

These and other objects and advantages will become more apparent from aconsideration of the following detailed description when read with theaccompanying drawings, wherein:

FIG. 1 is an elevational view, with parts broken away, of the switchincorporating the present invention;

FIG. 2 is a sectional view on line 22 of FIG. 1; and

FIG. 3 is a section through the switch taken on a plane parallel to thegeneral plane of FIG. I, viewed from below.

Referring now to FIG. 1, the switch of the present invention isgenerally indicated at 10. The main switch elements are shown as mountedon a base 11 within the broken away portion of a housing cover 12, thedetails of which are not important to the present discussion. Sufiice tosay that the support 11 and cover 12 are adapted to be mounted in themissile as described in the aformentioned Rimrner patent. Mounted onbase 11 is a contact support element 13, made of insulating material,which carries a plurality of auxiliary contacts Y14 and the stationarycontact 16 of the squib firing switch. The movable contact of the squibfiring switch is indicated at 17 and is carried by a rotatableinsulating disk 18 fixedly mounted on a shaft 19. Movable contact 17 isin electrical contact with a conductive segment 21 on the surface ofdisk 18 and the circuit continues through auxiliary contacts 14. It isto be understood that contacts 14 are spring fingers which actdownwardly against the surface of disk 18. Other conductive segments 22may be provided as desired on disk 18 to perform other independentfunctions.

Electrical leads, generally indicated at 25, extend from the variouscontacts to external circuitry, the leads extending from contacts 16 and17 being connected of course into the aforementioned squib firingcircuit.

Means are provided for rotating disk 18 to bring moving contact 17 intoengagement with stationary contact 16, which means are essentially thesame as described in the Rimmer patent. As mentioned above, disk 18 isfixedly attached to a shaft 19, which shaft is journalled for rotationin base 11 and carries two gears 23 and 24. Gear 23 is engaged androtated by a rack bar 26 which is biased to the right, as seen in FIG.3, by means of a spring 27. The rate of rotation of shaft 19 under theinfluence of spring 27 is controlled by an escapement mechanism 28,which engages gear 24. The details of escapement 28 are not shown, sincethey are not necessary to the understanding of the invention.

Prior to launching the missile, rack 26 is held in its retractedposition. For this purpose, a transverse bore 29 is provided in base 11in which a plunger 31 is guided for reciprocating motion. A flanged post32 mounted on base 11 coacts with plunger 31 to provide support for aspring 33 and to limit outward movement of plunger 31 by cooperationwith an annular plug 35 in plunger 31-. Plunger 31 is adapted. tocooperate with a ramp portion 34 on the end of rack bar 26. When plunger31 is pushed inwardly, ramp portion 34 is engaged by the end of theplunger to push rack bar 26 to the left as seen in FIG. 3 until theplunger clears the end of the rack bar. Plunger 31 is pushed in untilgroove 36 therein lines up with a slot 37 formed in base 11. An armingpin 38 is then inserted in the groove and slot in order to hold plunger31 in the inward position. When the missile is launched, pin 38 iswithdrawn, plunger 31 moves to the extended position, and rack 26-isfreed to rotate shaft 19 and bring contacts 16 and 17 together to closethe squib firing circuit after a predetermined time delay.

Both switch contacts 16 and 17 are formed as pins with circularcross-section, such a shape being easily machined and polished to a highdegree of smoothness. As stated above, such a polish is consideredessential to the reliable operation of this switch. The contacts may beeither solid or tubular, as long as the external surfaces are circularlycylindrical and polished. Further, as shown in the drawings, thecontacts are arranged with the planes containing their axes at 90 to oneanother, although it is to be understood that the precise angle is notcritical as long as it is large enough to insure substantially pointcontact between the contacts. A precious metal plating may be put on thecontact surfaces as desired.

When constructed with the polished crossed cylindrical contacts asdescribed above, the switch performed successfully when tested inaccordance with -9816. Moreover, it was found that the test could berepeated several times without lowering the performance, thus indicatinghigh reliability of performance in actual use.

It will be understood that the above description is ina stationarycontact physically attached to said housing means, said stationarycontact having ilow electrical resistance and a highly polishedcircularly cylindrical outside surface thereon,

a movable contact in said housing means having low electrical resistanceand a highly polished circularly cylindrical outside surface thereon,said movable contact having the plane containing its longitudinal axisdisposed at approximately a 90 angle with respect to the planecontaining the axis of said stationary contact, electrical leads havinga low resistance extending from said contacts and adapted to beconnected into said squib firing circuit, said leads and. contactscarrying current on the order of amperes when said movable contactcloses on said stationary contact and,

actuating means in said housing means having apre'-' determined timedelay moving said movable contact surface into engagement with saidstationary contact surface after said predetermined time delay.

References Cited by the Examiner UNITED STATES PATENTS 1,599,507 9/ 1926Abrahamson 200-33 2,653,199 9/1953 Brown et al 200-166 X: 2,755,3467/1956 Fisher 200-166 X 3,140,376 7/ 1964 Tilse, et al. 200-166 KATHLEENH. CLAFFY, Primary Examiner. H. O. JONES, Assistant Examiner. i

